Bird flu tops list of viruses likely to cause another pandemic - so authorities are on high alert

A sheep infected with bird flu will ring another alarm bell as the virus continues to drift into mammals.

It's the first time anywhere in the world that a sheep is known to have been infected. It just happens to have been in Yorkshire.

But the H5N1 virus is also found quite widely in US dairy cattle. And Sky News revealed in December that horses can harbour the virus too.

These are 'spill-over' infections, with the bird virus jumping species. That's a concern, particularly if the virus shows signs of adapting to mammalian cells.

And the close contact that humans have with domesticated animals is an extra worry because the virus has more opportunities to jump species again.

The UK is in another wave of H5N1, with large areas now part of avian influenza prevention zones. Millions of poultry have been culled in recent months.

Wild mammals are also being affected. Most recently the virus has been detected in dead seals in north Norfolk.

But foxes, otters, dolphins and porpoises have also been found to have the virus.

The Animal Plant Health Agency is closely monitoring the situation.

Ever since the outbreak in US dairy cattle, it has started routine testing of livestock on UK farms with infected poultry.

That's how this ewe was detected, with H5 antibodies in her blood and genetic material from the virus in her milk.

Fortunately, all the other sheep on the farm were negative, but the animal was culled.

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Swift action like that reduces the chances of the virus evolving genetic changes and spreading.

US authorities were too slow to act when the virus was detected in cattle.

It's now in at least 950 herds in 16 states, and harmless fragments of the virus have been detected in milk on supermarket shelves.

Further tests are being carried out on the virus that infected the Yorkshire sheep.

Scientists will look at whether it had any genetic changes compared to the variants found in birds.

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The UK Health Security Agency says H5N1 is still primarily a bird disease and the risk to the general public is still very low.

But it tops the list of viruses likely to cause a pandemic, so authorities are on high alert.